1. Field of the Invention
The present invention relates to a method for controlling an SRM (Switched Reluctance Motor) switching angle by an analog encoder system that can perform a precise control of the switching angle of the SRM by comparing an analog sensor signal detected by the analog encoder system with a switching-on/off command signal.
2. Description of the Prior Art
With the rapid technical development of power semiconductor devices, they now have a high switching speed and a large capacity. Also, with the spread of mechatronics in industry, the development of multi-function and high-performance motors has been actively made.
An SRM is a kind of single excited machine that has a simple structure and is low-priced. The SRM is stable against any short-through fault through its phase separation, and has the speed-torque characteristic of a DC motor. Additionally, the SRM has a wide speed variation range, superior high-speed and forward/reverse rotation characteristics, and a strong structure. The researches and developments for expanding the application fields of SRM into the fields of home appliances, electric vehicles, airplanes and the whole industry have now been in progress around advanced countries.
In the SRM, a rotor is forced in a direction that the magneto resistance of an excited magnetic circuit is minimized in order to produce a resultant rotating force. This corresponds to the physical phenomenon whereby the energy of a system is minimized through the change of the energy stored in the system to a mechanical energy. Devices using this physical principle include a pulling magnet as a simple actuator, a linear solenoid, a relay, a step switch, etc.
FIG. 1 is a view illustrating a conventional SRM driving system. The SRM is a kind of motor that uses reluctance torque. In order to use the reluctance torque at maximum, both a stator 10 and a rotor 20 of the SRM have a salient pole type structure, and wire is wound only on the stator 10 as a concentrated wire. In this case, the torque is generated in a direction that the reluctance of the magnetic circuit becomes minimized, and the magnitude of the torque generated accordingly is in proportion to the square of a current i flowing through an upper winding and the change rate of an inductance L to a position angle θ of the rotor 20 as shown in Equation (1.).                     T        =                              1            2                    ⁢                      i            2                    ⁢                                    ∂                              L                ⁡                                  (                                      θ                    ,                    i                                    )                                                                    ∂              θ                                                          (        1        )            
Accordingly, the torque generation section should be utilized at maximum by maximize the change rate of the inductance and promptly establishing the current corresponding to a load at the on/off time point of respective upper switches.
That is, the control of the SRM is affected by the input voltage and the switching-on/off angle, and in order for a voltage source to effectively raise the current, it is required to perform a switching operation before the salient poles of the stator and the rotor meet together. The switching-on angle is an important factor for properly raising the current. Accordingly, in order to obtain the optimum SRM operation characteristic, it is necessary to control the switching-on/off angle accurately.
As described above, the upper winding of the stator 10 of the SRM requires information about the position angle of the rotor 20 in view of the characteristic that it should be excited in synchronization with the position of the rotor 20. Although the detection of the position angle of the rotor 20 is generally performed using an encoder or a resolver, such a mechanical external position sensor has the problems in that as its resolution is higher, its manufacturing cost becomes higher. Accordingly, in order to reduce the burden of the installation cost, a low-priced encoder has conventionally been used. Additionally, in order to completely remove the encoder installation cost, researches for a sensorless SRM switching angle control method have actively been in progress.
In consideration of the cost, it is general to use a low-priced encoder, and especially an incremental encoder. The incremental encoder operates in a manner that the number of output pulses according to the position of the rotor is counted and converted into a digital value by an up/down counter, and a microprocessor controls signals of respective phases according to the digital counted values.
Generally, a digital code type encoder system having a microprocessor is widely used in motor control due to its characteristics of high performance, easy data process, programming flexibility, etc. In the case of the incremental encoder, the accuracy of the position and speed of the rotor depends upon the sampling period and the resolution of the encoder, and thus the control performance of the SRM is determined by the accuracy of the rotor position and the performance of the microprocessor.
The absolute measurement error is determined by an error (Δθe) according to the resolution of the encoder (expressed by Equation (2)) and an error (Δθp) according to the rotor speed during a sampling period (expressed by Equation (3)).                               Δθ          e                =                  2          ⁢          π          ⁢                                    N              r                                      N              p                                                          (        2        )            Δθp=ωr·Ts (3)
Here, Nr denotes the number of rotor poles, Np the resolution of the encoder, Ts the sampling period of the microprocessor, and ωr the rotating speed, respectively. Accordingly, the absolute measurement error is expressed by Equation (4).                               Δθ          s                =                                            Δθ              e                        +                          Δθ              p                                =                                    2              ⁢              π              ⁢                                                N                  r                                                  N                  p                                                      +                                          ω                r                            ·                              T                s                                                                        (        4        )            
FIG. 2 is a graph illustrating the absolute measurement error according to the speed in the microprocessor of the incremental encoder system. It can be seen that in a low-speed region, the deviation of the motor position in the sampling period is increased as the motor speed is increased. Generally, if the motor speed is 3000 [rpm] and the sampling period is 200 [μs] in a state that the encoder system uses 1024 pulses and the number of rotor poles is 8, the system may have an error of maximum electric angle of 31.6125 degrees. This causes the generation of torque ripples and the increase of the whole system cost.
In the case of an optical encoder, the control of the switching angle is performed by an output signal of the optical encoder arranged under inductance. FIGS. 3a and 3b illustrate conventional encoder disks and phase sensor signals. The optical encoder has a very simple structure with a low cost, but it is very difficult to perform a high-resolution control of the switching angle through the optical encoder. The switching-on/off angle is calculated at a rising edge and a falling edge of the optical encoder signal, and the accuracy of the calculated switching angle depends upon the microprocessor and the rotor speed. In this case, a PWM (Pulse-Width Modulation) technique is used for the torque adjustment, but the switching of a high frequency causes a switching loss.
Additionally, according to the above-described switching angle control method using the low-priced encoder, it is difficult to perform an accurate phase detection due to its limited resolution, and thus an optimum operation cannot be achieved over the whole operation range. Meanwhile, the SRM control system by the microprocessor has the problems in that its performance is restricted by not only the resolution of the encoder but also the sampling period of the microprocessor. In this case, as the operation speed of the motor becomes higher, the accuracy of the phase switching-on/off angle becomes lower to cause an unstable operation of the motor.